Casting methods for making articles having a fine equiaxed grain structure

ABSTRACT

Methods for casting a metallic material to form a component are described. The component can be a superalloy-containing turbine part, for example. The general method includes the step of pouring the metallic material, in molten form, into an investment mold; and then rapidly immersing the entire investment mold into a bath that contains a low-melting liquid coolant metal, so as to achieve substantially uniform, multi-directional heat transfer out of the molten material. The molten material that solidifies to form the component is characterized by a fine-grained, equiaxed grain structure. Related embodiments include the use of two ingots that constitute the superalloy material. One ingot includes the oxygen-reactive elements, and is prepared by a vacuum-melting technique. The other ingot includes the remainder of the elements, and can be prepared by a number of techniques, such as air-melting processes.

BACKGROUND

This disclosure is generally related to metallic components, and methods for manufacturing those components. In some specific embodiments, the disclosure is related to cast metallic articles, often formed from nickel- or cobalt-based superalloys; and related, specialized casting methods.

A number of metals and metal alloys are employed in demanding applications, in terms of strength, oxidation resistance, and/or high temperature resistance. Examples include titanium, vanadium, molybdenum, and superalloys based on nickel, cobalt, or iron. The superalloys are especially suitable for high-temperature applications, such as, for example, gas turbine engine components of aircraft engines and power generation equipment. Very often, these components are manufactured by casting processes, such as investment-casting. While metal casting has been practiced for thousands of years, the techniques have become quite sophisticated in modern times, due in part to the high level of integrity required for cast parts such as jet engine blades.

The integrity and overall quality of the metal component is determined in part by its crystalline structure, e.g., the grain size and orientation of the grains in the component. The desired grain structure is, in turn, often dependent on the projected operating temperature of the part. As an example in the case of gas turbine components formed from various superalloys, the turbine blades (buckets) in the combustor of the turbine may be exposed to temperatures as high as 900-1150° C. These components usually have a directionally solidified (DS) columnar grain structure, or a single crystal structure, to resist high-temperature creep failure and other degrading effects.

In contrast, engine components that are subjected to lower operating temperatures often benefit from a very different grain structure. For example, gas turbine wheels and discs, while having their own set of performance requirements, often operate at temperatures of about 650-700° C. In many cases, it is very desirable that these components have a fine equiaxed grain structure.

Although fine equiaxed grain structures are commonly obtained in small castings, they are relatively difficult to produce in large, complex parts, such as the gas turbine airfoils and structural components. The investment casting techniques typically produce cast components having a mixture of columnar and equiaxed grains. This is often the case for large components with thick sections (e.g., sections more than about 10 mm thick). Obtaining the desired fine-grain structure can be especially difficult if the component has a complex geometry, with a wide variation in sectional thickness.

Non-uniform grain morphology and grain size can lead to problems in the quality and performance of the cast components. In many cases (though not all), large grain size can result in low strength at a given operating temperature. Moreover, a columnar grain structure—while desirable for components operating under a specific temperature regime—can be detrimental for the lower-temperature components referenced above. Columnar grain morphology is characterized by continuous, intergranular boundaries, along which cracks and “hot tears” can sometimes develop. Also, when oriented transversely to the stress-direction during use, the columnar grain boundaries can be weak, which can in turn lead to premature failure of the component.

With these general considerations in mind, new methods for casting high-performance alloys would be welcome in the art. The techniques should be especially suitable for manufacturing components that require a fine, equiaxed grain structure. Moreover, the new developments should also be suitable for casting relatively large components having complex geometries. Furthermore, the techniques should not require substantial changes to current casting operations that would result in significant increases in manufacturing costs.

SUMMARY OF THE INVENTION

An embodiment of this invention is directed to a method for casting a metallic material to form a component, comprising the following steps:

(a) pouring the metallic material, in molten form, into an investment mold; and

(b) rapidly immersing the entire investment mold into a bath comprising a low-melting liquid coolant metal, so as to achieve substantially uniform, multi-directional heat transfer out of the molten material, thereby solidifying the molten material to form the component, and providing a fine-grained, equiaxed grain structure thereto.

An additional embodiment of this invention relates to a method of casting a nickel-based superalloy to form a turbine engine component. The method includes these steps:

(i) preparing a first ingot by a vacuum-melting technique, wherein the first ingot comprises nickel and all elements in the superalloy that are oxygen-reactive; and

-   -   preparing a second ingot by either an air-melting technique, an         inert gas technique, or a vacuum-melting technique, wherein the         second ingot comprises the superalloy elements that are         generally non-reactive with oxygen;

(ii) attaching the two ingots together, or placing the two ingots together, to form a casting charge; and melting the charge and pouring the molten material into an investment mold; and

(iii) rapidly immersing the entire investment mold into a bath comprising a low-melting liquid coolant metal, so as to achieve substantially uniform, multi-directional heat transfer out of the molten material, thereby solidifying the molten material to form the component, and providing a fine-grained, equiaxed grain structure thereto.

DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 is a photograph of an exemplary wax mold structure, suitable for some of the casting embodiments of the invention.

FIG. 2 is a photograph of an exemplary ceramic mold structure, based on the wax mold structure of FIG. 1.

FIGS. 3A and 3B are cross-sectional schematics of an exemplary casting system for embodiments of this invention, prior to and after immersion of the associated shell mold into a coolant bath.

FIG. 4 is an illustration of two ingots joined to each other, each containing particular constituents of a nickel-based superalloy.

FIG. 5 is a photomicrograph of an etched section of a nickel-based superalloy sample, after being cast according to embodiments of this invention.

DETAILED DESCRIPTION

In regard to this disclosure, any ranges disclosed herein are inclusive and combinable (e.g., compositional ranges of “up to about 25 wt %”, or more specifically, “about 5 wt % to about 20 wt %”, are inclusive of the endpoints and all intermediate values of the ranges). Moreover, the terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Moreover, approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.

As alluded to previously, a number of metals can be cast according to embodiments of this invention. Examples include the “superalloys”, a term intended to embrace iron-, cobalt-, or nickel-based alloys. The superalloys usually include one or more additional elements to enhance their high-temperature performance. Non-limiting examples of the additional elements include cobalt, chromium, aluminum, tungsten, molybdenum, rhenium, ruthenium, zirconium, carbon, titanium, tantalum, niobium, hafnium, boron, silicon, yttrium, and the rare earth metals. (Each of the base alloys may contain one or more of the other elements listed as base alloys, e.g., nickel-based alloys containing cobalt and/or iron). Other metals that can be cast according to this invention include titanium or titanium alloys, or stainless steel alloys.

In a typical embodiment of this invention, the metal or metallic alloy being used to form the component is initially in the form of powder, particulates, or ingots. The material is then heated to a sufficient melting temperature. (In the case of nickel-containing alloys, the alloy is usually heated to about 1350° C. to about 1750° C.). The molten metal may then be poured into a mold in a casting process, to produce the desired shape.

In most embodiments, investment casting is generally used to make the parts. This technique is very useful for producing large parts, and/or parts that have complex shapes, and which must be capable of withstanding high temperatures. The investment mold is usually made by making a pattern, using wax or another material that can be melted away. This wax pattern is dipped in refractory slurry, and then coated with a refractory sand which coats the pattern and forms a shell. The shell is dried, and the process of dipping in the slurry, sanding and drying is repeated until a robust thickness is achieved. After these steps, the “shelled” pattern is placed in an autoclave, and the wax is melted away. The empty shell is then fired, resulting in a mold that can be filled with the molten metal/alloy.

Since the shell mold is usually formed around a one-piece pattern, (which does not have to be pulled out from the mold as in a traditional sand casting process), very intricate parts and undercuts can be made. The wax pattern itself is usually made by duplication, e.g., with an injection die, or using stereolithography. The master pattern can be fabricated by various techniques, e.g., using a computer solid model master. Many variations of the overall process are known to those skilled in the art.

Preferably, the investment mold is pre-heated to about 800-1100° C., to remove any residues of wax, as well as to harden any binders that are present. The pre-heating step is usually carried out in a vacuum, or in an inert atmosphere, but can be carried out in air as well. Investment casting techniques are described in many references, such as “Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure, and Properties”, by T. Pollock et al; and the Journal of Propulsion and Power, Vol. 22, No. 2, March-April 2006. The metallic material can be directed to the mold by a variety of pouring techniques. As examples, pouring can be carried out by using gravity, pressure, inert gas, or vacuum conditions. The preferred embodiment in some instances is to cast in vacuum.

In some embodiments, a nucleating agent is disposed on the interior surface of the mold. The nucleating agent—even at small concentrations—can enhance the formation of the highly-desirable, equiaxed grain structure. A number of compounds and materials can serve as a nucleating agent (sometimes referred to as a “grain refining agent”). Non-limiting examples include metals such as cobalt, ruthenium, rhodium, iridium, and platinum; or oxides of nickel, cobalt, or iron. Mixed oxides of silica, cobalt, and alumina can also be used, for example. In some preferred embodiments, the nucleating agent comprises at least one cobalt-containing oxide, e.g., cobalt aluminate. The use of nucleating agents for some casting applications is described, for example, in U.S. Pat. No. 5,823,243 (T. Kelly), incorporated herein by reference.

In a typical embodiment, the nucleating agent can be incorporated into the interior surface of the mold in the form of a facecoat. The facecoat can be applied in any convenient manner. Alternatively, the nucleating agent can be applied as a “wash” coating to the surface of the mold.

The concentration of the nucleating agent will depend on various factors, such as the type of metal being cast; and the identity of the particular nucleant. In some embodiments, e.g., using cobalt aluminate when casting a nickel-based alloy, the final concentration of the nucleant will be in the range of about 5% to about 10%, by weight, in a wash coating composition. However, the concentration could be much higher, depending on the factors noted above. Related information on the use of nucleants can also be found in “Microstructure and Mechanical Properties of High Temperature Creep Resisting Superalloy Rene™ 77 Modified CoAl₂O₄”, by M. Zielinswka et al; Archives of Materials Science and Engineering; Vol. 28; Issue 10; October 2007; pp. 629-632, which is incorporated herein by reference.

The bath used to provide cooling to the filled mold (as further described below) can be in a variety of different shapes. The bath can be contained in a tank that may be equipped with cooling coils. Heating elements may also be included, to adjust the bath temperature. Moreover, a suitable stirring mechanism may be provided to assure circulation of the liquid bath. As also described below, the volume of the bath is calculated to accommodate a selected number of investment molds, according to an arrangement that maximizes a specific cooling regimen.

Low-melting liquid coolant metals for the bath are known in the art. In many embodiments, the cooling liquid is either tin or aluminum. Tin is especially preferred because of its low melting temperature and low vapor pressure. In general, the liquid coolant metal is maintained at a temperature as low as practically possible, e.g., a temperature not much greater than the melting point of the metal. In some typical embodiments, the liquid coolant metal is maintained at a temperature of about 700° C. to about 1400° C. below the melting point of the metallic material being cast, while the metallic material is poured into the mold. In the case of nickel-based superalloys that are being cast, a suitable temperature for a tin bath is often between about 250° C. and about 350° C. The metallic material being cast is usually poured into the mold (which is pre-heated), at a material temperature of about 50° C. to about 150° C. above its melting point.

As further described below, in some detail, and depicted in the figures, the investment mold is rapidly immersed in the bath after the mold has been filled with the molten metal. The present inventors discovered that rapid cooling within the bath can dramatically influence the microstructure of the cooled casting. As described below, a fine, equiaxed grain structure can result, which in turn provides other important properties to the cast article. (In some embodiments, the grains are substantially uniform as well).

In some embodiments, the molten metallic material that is poured into the mold includes a relatively small amount of dispersed solid particles of the metallic material. The solid particles can provide additional grain refinement for the final casting. Usually, the solid particles comprise less than about 2% of the total weight of the metallic material being cast; and in some embodiments, less than about 1%. According to one exemplary technique, the solid particles can be provided by subjecting the molten casting material to at least one solidification-melting cycle, prior to pouring the molten material into the shell mold. A “melting-freezing” sequence can induce the formation of the particles within the body or depth of the casting material, prior to re-melting as the material is poured into the mold.

FIG. 1 is a depiction of an exemplary wax mold structure 10 (in simplified form), suitable for embodiments of the invention. The structure includes a pour cup 12 formed of wax, and supported by a number of ceramic support posts 14. The pour cup is designed to contain the molten metal to be used in the casting. The lower region of the pour cup 12 communicates with a number of feed tubes or “runners” 16, also formed of wax. Each of the feed tubes 16 terminates in an interior region of a wax article mold 18.

The molds 18 have previously been formed by known techniques to accommodate the precise shape of a part to be cast according to the process. The molds 18 and the posts 14 can be supported on the upper surface 20 of a base plate 22. Other general details regarding this type of wax mold structure are provided in a number of references, including U.S. Pat. No. 5,072,771 (Prasad); and “Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure, and Properties”, by T. Pollock et al; Journal of Propulsion and Power; Vol. 22, No. 2, March-April 2006, mentioned above. Moreover, other information regarding pour cup structures and details for supplying the molten metal to the mold can be found in various sources, e.g., U.S. Pat. No. 6,019,158 (Soderstrom et al). All three of these documents are incorporated herein by reference.

In situations where multiple molds (i.e., for multiple components being cast simultaneously) are to be immersed in the coolant bath, care must be taken to ensure the development of the fine equiaxed grain structure. The present inventors discovered that, in some embodiments, it is preferable that the investment molds be arranged, in the wax mold structure, so as to provide maximum, multi-directional heat transfer out of the molten material, once the structure is filled with molten metal and immersed in the bath. In some embodiments, the investment molds are arranged in a general, star-shape, relative to each other, along the longest access of each shell mold.

In other embodiments, where the shell molds might have a relatively high aspect ratio, the longest face or surface of each shell mold within the coolant bath is spaced from, and generally parallel to, the longest surface of at least one other shell mold. As one example, pairs of shell molds may have a facing surface opposite each other, as in FIG. 1. The shell molds may generally be situated upright, around a perimeter of the base plate of the mold structure. In general, the surfaces of the shell molds should be spaced in a manner to provide maximum heat transfer, as described above.

FIG. 2 is a photograph of an exemplary ceramic mold structure 30, based on the pattern of the wax mold structure 10, depicted in FIG. 1. As those skilled in the art understand, the ceramic mold is typically formed from a material such as alumina, silica, and/or zirconia. Typically, the mold is fabricated by the progressive build-up of the ceramic layers around a wax pattern like that of FIG. 1. Ceramic cores can be embedded in the wax to obtain internal structures, like cooling pathways. (In this figure, features not specifically marked are the same as those in FIG. 1, in ceramic form).

FIGS. 1 and 2 are generally described in terms of a single shell mold (e.g., shell mold structure 30). However, the figures can alternatively be described in terms of multiple shell molds, each “fed” by a feed tube 16 (FIG. 1), and each associated with a cast article formed in one of the specific wax molds 18. The concept of multiple molds is expressed here in terms of a single “mold structure”, so as to simplify the overall description.

Thermal cycles are employed to remove the wax material on which the ceramic layers were deposited, to form the ceramic structure of FIG. 2. Thus, the pour cup, feed tubes, and article molds (i.e., article molds 18 of FIG. 1) are all part of the ceramic pattern at this stage. The ceramic mold structure is usually supported on a base plate (chill plate) 32, as shown in FIG. 2. The mold structure (referred to as an “investment mold”) can now be filled with the molten metallic material, as part of the casting process.

The thickness of the mold in FIG. 2 can vary to some extent. As a general, non-limiting illustration for articles being cast from a nickel-based superalloy, and having an average thickness of about 10 mm to about 25 mm, the average thickness of an alumina-based mold will be in the range of about 5 mm to about 25 mm. (The overall mold size is also limited by the size of the furnace used in the casting process).

In some embodiments, the mold may be thicker in the upper region (i.e., the pour cup region); and in the lower region (i.e., the base plate region), relative to the central region, i.e., the region encompassing the cast part itself. (This can be viewed as the “critical cooling region”). The present inventors discovered that this difference in thickness can be especially important for effectively obtaining the desired microstructure, based on the required strength and integrity of the mold, balanced against the capacity of the mold for transferring heat out of the mold walls during the critical cooling process.

Thus, in some embodiments, the thickness of the mold in the cast part (central) region should be at least about 25% less than the thickness in both the pour cup region and the base plate region. In some preferred embodiments, the thickness of the shell mold in the cast part region is at least about 50% less than the thickness of the mold in the other regions. (The thickness of the mold in the pour cup region and the base plate region need not be identical to each other. Moreover, the thickness in the pour cup region is sometimes graded to some degree).

FIG. 3A is a simplified depiction of an exemplary casting system 50 for embodiments of this invention. A shell mold 52 (similar to that of FIG. 2), having at least one section shaped in the form of a desired casting (e.g., a turbine blade), is secured to a chill plate 54. For the purpose of depicting aspects of this invention, the mold is depicted in an upper position in FIG. 3A. FIG. 3B, described below, depicts the lower position, in which the mold has been immersed in the coolant bath.

An ingot of the casting metal can be placed within a crucible that is partially surrounded by a water-cooled induction coil. (The ingot and crucible are conventional features not specifically shown in the figure. Reference is made, for example, to the Pollock article mentioned previously). Other types of heating techniques are also possible.

The crucible that contains the ingot can be lowered into a furnace 62 by any suitable mechanical means. One example is the mechanical arm 64 that can be connected to a conventional drive system. The lower end of the arm can be connected to a platform supporting the mold and chill plate, or to any associated structure. In this manner, the vertical motion of the investment mold can be precisely controlled. The overall process is preferably carried out in a vacuum or in an inert atmosphere. An ambient air atmosphere can also be used, alone or in conjunction with the other environment(s), as a form of cooling the mold after withdrawal from the heating chamber.

The casting system also includes some means (not specifically shown) for preheating the ceramic shell mold 52 to a suitable temperature, usually above the liquidus temperature of the casting metal or metal alloy. The system can further include a baffle 66, situated between a lower region of the furnace 62 and an upper region of the coolant bath 68. The baffle can assist in obtaining a steep thermal gradient between the superheated mold and the cooling liquid bath. The baffle may be in the form of a single layer or multiple layers, and usually (though not always) comprises a stiff or flexible thermal insulating material. The baffle may be rigid, or may float. In the illustrative embodiment of FIG. 3A, an additional floating baffle 69 is also provided. In general, the baffle 66 can be designed to vary its fit around the shape of the mold as the mold is withdrawn from the heating chamber, through the baffle(s), and into the liquid coolant bath 68.

The general function of pour cup 71 was described in reference to FIGS. 1 and 2. The pour cup receives the molten casting metal 73 from a crucible (not shown) that is generally situated above the cup, and communicates therewith. The particular design and features of the pour cup are not critical to this invention.

The coolant bath 68 contains a suitable coolant metal, as mentioned previously, and is usually equipped with a stirrer (not shown), or other means of agitation. The bath is often surrounded by a liner 72 of thermal oil, that can be used to control the temperature of the bath. The thermal oil in the liner can be circulated through in-flow and out-flow conduits, as shown in the Pollock article.

The size and shape of the bath can vary somewhat. Various factors are involved, such as the size and type of castings involved; as well as the type of furnace used; and general space requirements. In some embodiments, the bath has a mass that is at least 4 times (and preferably greater than 4 times) the combined mass of the cast metal and its mold. (The figures in this disclosure are not necessarily drawn to scale, so that various features can be highlighted to the reader).

After the molten casting metal has been directed into shell mold 52, the mold is rapidly immersed into coolant bath 68, as depicted in FIG. 3B. As mentioned above, rapid immersion was found to provide a fine, equiaxed grain structure. As used herein, a “fine, equiaxed grain structure” refers to a population of grains having a median aspect ratio of less than about 2.5. In some embodiments, the cast material is characterized by an average grain diameter of about 3 mm or less.

Rapid immersion can be characterized as a “withdrawal rate”, i.e., the withdrawal of the mold from the hot zone (furnace 62) of the casting system. Since the investment mold moves from the furnace to the bath with very little residence time in-between, the withdrawal rate is effectively the “plunge rate” or “quench rate” for moving the mold entirely into coolant bath 68. The particular withdrawal rate will depend on various factors, such as the identity of the casting metal, and its projected size and shape; the liquidus temperature of the casting metal; the coolant bath temperature and bath size; the wall thickness and overall size of the investment mold; and the presence or absence (and type) of any nucleating agent.

In the case of a nickel- or cobalt-based superalloy being cast, wherein the shell mold thickness (in the cast part region) is in the range of about 3.5 mm (0.14 inch) to about 1 cm (0.4 inch); and wherein a cobalt-based nucleating agent has been incorporated into the mold surface, the withdrawal rate is usually at least about 380 cm (150 inches) per hour, e.g., in the range of about 380 cm (150 inches) to about 510 cm (200 inches) per hour. In some specific embodiments, the withdrawal rate is greater than about 510 cm (200 inches) per hour. The relatively high withdrawal rate minimizes the amount of radiational cooling experienced by the shell mold, i.e., between the furnace and coolant bath; and this can also enhance formation of the desired grain structure. In some embodiments directed to a conventional turbine engine component, the entire mold is immersed in the bath within about 300 seconds after the metallic material has been poured into the mold.

In the case of titanium-based alloys, the withdrawal rate is usually at least about 380 cm (150 inches) per hour, and in some preferred embodiments, at least about 510 cm (200 inches) per hour. In the case of stainless steel alloys, the withdrawal rate is usually at least about 380 cm (150 inches) per hour, and in some preferred embodiments, at least about 510 cm (200 inches) per hour. Those skilled in the casting arts will be able to determine the most appropriate withdrawal rate for a given situation; based on the teachings herein.

It should be emphasized that withdrawal rates for other casting and solidification processes are, often, considerably longer than those described above for embodiments of the present invention. An example can be provided for an article formed of a nickel- or cobalt-based superalloy having dimensions similar to articles described herein, and subjected to a direct solidification (DS) process. In that technique, the article may be withdrawn (i.e., immersed in the coolant bath) at a rate less than about 125 cm (50 inches) per hour. The resulting microstructure of such an article is usually very different from the fine-grained, equiaxed grain structure described herein.

After casting is complete, conventional steps are undertaken to release the cast article. Typically, the investment mold is separated from the article by hammering, media blasting, vibration, water-jetting, chemical dissolution, or some combination of these techniques. The sprues or gates used in the molding process are cut off, and the casting can then be subjected to other cleaning and finishing steps, such as grinding.

The quality of the casting alloy, and the cost of preparing and using such an alloy, can be very important factors in manufacturing high-performance components for the present invention. Vacuum melting techniques, such as vacuum-induction melting and vacuum-arc remelting, can be used to produce high-quality ingots of an alloy composition. These types of techniques are often required when preparing alloys that include oxygen-reactive elements, like aluminum, titanium, and zirconium. However, in terms of equipment and operating details, the techniques can be relatively expensive to employ—especially in the case of very large ingots of alloy material, e.g., about 450 kilograms (1000 pounds) or more. In many embodiments for this invention, the alloys do in fact include at least one of the oxygen-reactive elements.

On the other hand, various air-melting techniques, such as argon oxygen decarburization, are very attractive for preparing alloys, e.g., wrought alloys used for plate, sheet and bar tube. (The air-melting technique is also sometimes used as a preliminary step to subsequent vacuum processes). The popularity of the air-melting techniques is based in part on their lower cost, as compared to vacuum melting. However, the air-melting techniques cannot readily be undertaken when the desired alloy contains the oxygen-reactive elements mentioned above.

Thus, according to embodiments of this invention in which the metallic material contains both reactive and unreactive elements, two separate casting compositions, e.g., ingots, are first prepared. The first ingot comprises the elements that are generally reactive with oxygen. As noted above, the reactive elements usually comprise at least one of aluminum, titanium, zirconium, hafnium, and the rare earth metals. In the case of a superalloy, the first ingot may also comprise at least one of the base elements, i.e., nickel, cobalt, or iron. The first ingot may also comprise at least one of carbon, boron, silicon, and in some cases, tantalum. The weight-ratio of base metal elements (total) to oxidation-reactive elements (total) in the first ingot will usually be in the range of about 90:10 to about 80:10.

The first ingot can be prepared by a vacuum-melting technique. As alluded to previously, those techniques are known in the art. In some preferred embodiments, the vacuum-melting technique is either induction melting or non-consumable arc melting.

The second ingot comprises the elements that are generally non-reactive with oxygen. In the case of the superalloy composition, the second ingot comprises at least one of nickel, cobalt; and iron; and at least one of chromium, molybdenum, tungsten, rhenium, and in some cases, tantalum. In preferred embodiments, the composition of the second ingot is substantially free of oxidation-reactive elements such as aluminum, zirconium, and hafnium. In other embodiments (though not all embodiments), the second ingot is also substantially free of at least one of carbon or boron.

The second ingot can advantageously be prepared (especially from an economic point of view) by any suitable air-melting technique, such as air casting or argon oxygen decarburization. However, in other embodiments, it is also possible to prepare the second ingot by any of the suitable vacuum melting techniques described herein. Moreover, an inert gas technique can sometimes be used for the second ingot.

The proportion of elements within each ingot, and relative to the other ingot, will depend in large part on the required composition for the final casting. Other factors include the particular melting techniques used for each ingot, as well as the relative melting points of the two alloys. (In terms of formulating the ingot-compositions, the melting point of each ingot should not be so high that the melting step can be a difficult one. Moreover, it may sometimes be desirable to “space” the melting point of one ingot from the other, in order to enhance grain refinement in the final, cast alloy). The weight of each ingot relative to the other ingot will also depend on the factors noted above, along with the intended, final metal composition. Other details regarding these types of ingots can be found in U.S. Pat. No. 4,718,940 (McPhillips), which is incorporated herein by reference.

The two ingots can then be joined together by any convenient technique, such as spot welding or any type of mechanical attachment. Alternatively, they may simply be placed together or stacked on top of each other. FIG. 4 exemplifies a nickel-based superalloy, and provides a simple depiction of a first ingot “A”, joined to a second ingot “B”. The first ingot contains nickel and oxidation-reactive elements; while the second ingot contains nickel and the remainder of the typical superalloy constituents. The relative weight percentages noted in the figure are illustrative, and can vary, as mentioned above.

In general, there is at least one economic advantage in using the two ingots, when the bulk of the material in ingot “B” is formed by the less expensive air-melting technique. The ingot “A”, while being prepared by the more expensive vacuum melting technique, requires a less costly preparation because of its relatively small size. Moreover, problems that might otherwise arise in processing the oxidative-reactive elements are minimized or prevented.

The two ingots can remain joined or next to each other until they are needed to produce the desired investment-cast component. They can then be added to a crucible within a system like that of FIG. 1, for carrying out the casting process. The present inventors discovered that, in addition to providing important economic-processing advantages in some instances, the use of the two ingots in the described technique also can result in further enhancement in the fine-grained, equiaxed structure.

A large number of components can be cast according to embodiments of this invention. In general, any component formed of high-temperature/high-strength materials like the superalloys, titanium alloys, and stainless steel alloys, can benefit from this invention. Non-limiting examples of turbine engine components that can be formed as described herein include turbine buckets, blades, vanes, nozzles, combustor liners, combustor domes, and shrouds. The inventive processes described herein are especially useful for components having a mixture of columnar and equiaxed grains. This is often the case for “large” components, as described above, (e.g., those with sections having a thickness greater than about 10 mm), since obtaining the fine-grained crystal structure for those sized components has been traditionally very difficult.

In contrast to components formed of alloys without the fine, equiaxed grain structure, the microstructure of components formed by embodiments of this invention can be very desirable. An illustration can be provided in the case of turbine engine components formed of nickel-based superalloys. In those cases in which the alloys have the fine, equiaxed microstructure, the components are expected to exhibit a yield strength that is at least about 30% higher than that of a component formed of a cast alloy having a relatively coarse-grained structure.

EXAMPLE

An ingot of a nickel-based superalloy, Rene™ 108, was used in this example. The alloy had an approximate composition as described in U.S. Pat. No. 5,897,801 (Smashey et al), incorporated herein by reference. The ingot was melted in a crucible at 1400° C., using an apparatus similar to that depicted in FIG. 3A, and using a Ni-30Cr plug. The molten metal was directed into an alumina-based mold that had an average mold wall thickness (in the casting part region) of about 0.5 cm (200 mils). Prior to receiving the metal, a facecoat that contained about 5-10% by weight CoAl₂O₄ was coated onto the interior surface of the mold. The mold had been pre-heated to 1250° C.

After the molten alloy filled the mold, the mold was rapidly immersed in a liquid tin bath maintained at a temperature of about 250° C. The withdrawal rate (i.e., immersion speed) was (590 cm) (233 inches) per hour. After solidification of the cast part (an elongate test plate), the part was removed from the mold, cleaned, and then etched in a standard, acid-containing etching solution.

FIG. 5 is a photomicrograph showing the microstructure of a cross-section of the alloy, after etching. The average grain size was about 300 microns. The grains had a median aspect ratio of less than about 2.5. These measurements indicate that the microstructure was considered to be fine-grained and equiaxed, according to embodiments of this invention.

While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. 

What is claimed:
 1. A method of casting a metallic material to form a component, comprising the following steps: (a) pouring the metallic material, in molten form, into an investment mold; and (b) rapidly immersing the entire investment mold into a bath comprising a low-melting liquid coolant metal, so as to achieve substantially uniform, multi-directional heat transfer out of the molten material, thereby solidifying the molten material to form the component, and providing a fine-grained, equiaxed grain structure thereto.
 2. The method of claim 1, wherein the investment mold is pre-heated, in a vacuum or an inert atmosphere.
 3. The method of claim 1, wherein the investment mold comprises an interior surface that includes a nucleating agent for enhancing the formation of the equiaxed grain structure.
 4. The method of claim 3, wherein the nucleating agent comprises at least one cobalt-containing oxide.
 5. The method of claim 1, wherein the bath has a mass that is at least 4 times the total mass of the mold and the cast metal.
 6. The method of claim 1, wherein the rate of immersion is defined by a withdrawal rate of at least about 380 cm (150 inches) per hour.
 7. The method of claim 1, wherein the metallic material is at a temperature of about 50° C. to about 100° C. above its melting point, while being poured into the mold.
 8. The method of claim 1, wherein the liquid coolant metal is at a temperature of about 700° C. to about 1400° C. below the melting point of the metallic material, while the metallic material is poured into the mold.
 9. The method of claim 1, wherein the molten metallic material being poured into the mold includes dispersed solid particles of the metallic material; and the solid particles comprise less than about 2% of the total weight of the metallic material.
 10. The method of claim 9, wherein the solid particles within the molten material are obtained by subjecting the molten material to at least one solidification-melting cycle, prior to pouring the molten material into the mold.
 11. The method of claim 1, wherein multiple investment molds are immersed in the bath to form multiple components; and the investment molds are arranged in the bath to provide maximum, multi-directional heat transfer out of the molten material.
 12. The method of claim 11, wherein the investment molds are arranged in a general star-shape, relative to each other, along the longest access of each mold.
 13. The method of claim 1, wherein the metallic material comprises a superalloy based on nickel, cobalt, iron, or combinations thereof.
 14. The method of claim 13, wherein the component is a turbine engine part.
 15. The method of claim 1, wherein the metallic material for the component comprises a group of elements generally unreactive with oxygen; and also comprises at least one oxygen-reactive element.
 16. The method of claim 15, wherein the metallic material to be cast is produced by preparing a first ingot by a vacuum-melting technique, wherein the first ingot comprises all of the oxygen-reactive elements and at least one base element selected from nickel, cobalt, or iron; preparing a second ingot by either an air-melting technique, an inert gas technique, or a vacuum-melting technique, wherein the second ingot comprises all of the generally unreactive elements; attaching the two ingots together, or placing the two ingots together, to form a casting charge; and melting the charge and pouring the molten material into the investment mold.
 17. The method of claim 16, wherein the vacuum-melting technique is selected from the group consisting of vacuum induction melting; vacuum arc re-melting; and non-consumable arc melting; and the air-melting technique is selected from the group consisting of air casting and argon oxygen decarburization.
 18. The method of claim 16; wherein the first ingot comprises at least one of nickel, cobalt; and iron; and at least one of aluminum, titanium, zirconium, hafnium, and the rare earth metals.
 19. A method of casting a nickel-based superalloy to form a turbine engine component, comprising the steps of: (i) preparing a first ingot by a vacuum-melting technique, wherein the first ingot comprises nickel and all elements in the superalloy that are oxygen-reactive; and preparing a second ingot by either an air-melting technique, an inert gas technique, or a vacuum-melting technique, wherein the second ingot comprises the superalloy elements that are generally non-reactive with oxygen; (ii) attaching the two ingots together, or placing the two ingots together, to form a casting charge; and melting the charge and pouring the molten material into an investment mold; and (iii) rapidly immersing the entire investment mold into a bath comprising a low-melting liquid coolant metal, so as to achieve substantially uniform, multi-directional heat transfer out of the molten material, thereby solidifying the molten material to form the component, and providing a fine-grained, equiaxed grain structure thereto.
 20. The method of claim 19, wherein the investment mold comprises an interior surface that includes a nucleating agent for enhancing the formation of the equiaxed grain structure. 